This invention relates to apparatus for melting and dispensing thermoplastic materials such as hot melt adhesives. This type of thermoplastic material is received in the form of an elongated, flexible rod, as disclosed, for example, in U.S. Pat. No. 2,874,084, issued Feb. 17, 1959, in the name of Hans C. Paulsen. The invention disclosed may be utilized in portable hand-operated cement extruding guns such as used for home shop use or light industrial use and disclosed, for instance, in U.S. Pat. No. 3,743,142, issued July 3, 1973, in the names of Richard M. Elliott and Albert E. Newton. The invention may also be adapted to heavier industrial usage, for example, in applying adhesives in the manufacture of shoes or in the production of package containers.
In general, the glue dispensing appliance in which the subject system is used consists of a gun shaped device having a handle and a barrel. A heat radiating body (melt body) is constructed within the barrel to transfer heat from electric heaters to a melt chamber. Means are provided in the handle to feed a flexible rod of heat activatable glue to the melt chamber. As the glue is melted it is forced out of a nozzle at the exit of the melt chamber. A thermostat or other heat sensing device is used to prevent overheating and maintain the desired heat temperature of the melt chamber.
In the normal operation of this type of appliance, the heaters are energized and allowed to reach operating temperature. The flow of glue is generally initiated intermittently as needed without shutting the power off between uses. The glue gun may, ther1fore, sit idle for extended periods while the desired temperature is maintained by a suitable sensor-control. However, when the glue feed is actuated, an immediate demand for more heat occurs because of the relatively low temperature of the incoming glue. The normal control senses the temperature of the melt body, not the temperature of the glue, in the melt chamber. This causes a delay in response and a resultant drop in output glue temperature.
Even with optimum heat transfer characteristics, the melt chamber will not be able to heat the flowing glue to its identical temperature and it has been found in this case that while glue is flowing, there will be approximately a 60.degree. differential between the temperatures of the melt chamber and the output glue. A quick way to compensate for this heat loss would be to raise the temperature of the melt body 60.degree. beyond the desired glue temperature. However, this would overheat the residual glue left in the chamber during the idle condition and cause a loss in bonding strength.
It is therefore the object of this invention to provide a control system for the melt body temperature which maintains the glue as close as possible to the temperature of the melt body without causing the temperature of the glue to exceed the desired output temperature.